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The Serializer Component

The Serializer Component

The Serializer component is meant to be used to turn objects into a specific format (XML, JSON, YAML, ...) and the other way around.

In order to do so, the Serializer component follows the following simple schema.

../_images/serializer_workflow.png

As you can see in the picture above, an array is used as a man in the middle. This way, Encoders will only deal with turning specific formats into arrays and vice versa. The same way, Normalizers will deal with turning specific objects into arrays and vice versa.

Serialization is a complex topic. This component may not cover all your use cases out of the box, but it can be useful for developing tools to serialize and deserialize your objects.

Installation

You can install the component in 2 different ways:

Then, require the vendor/autoload.php file to enable the autoloading mechanism provided by Composer. Otherwise, your application won't be able to find the classes of this Symfony component.

To use the ObjectNormalizer, the PropertyAccess component must also be installed.

Usage

Using the Serializer component is really simple. You just need to set up the Serializer specifying which encoders and normalizer are going to be available:

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use Symfony\Component\Serializer\Serializer;
use Symfony\Component\Serializer\Encoder\XmlEncoder;
use Symfony\Component\Serializer\Encoder\JsonEncoder;
use Symfony\Component\Serializer\Normalizer\ObjectNormalizer;

$encoders = array(new XmlEncoder(), new JsonEncoder());
$normalizers = array(new ObjectNormalizer());

$serializer = new Serializer($normalizers, $encoders);

The preferred normalizer is the ObjectNormalizer, but other normalizers are available. All the examples shown below use the ObjectNormalizer.

Serializing an Object

For the sake of this example, assume the following class already exists in your project:

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namespace Acme;

class Person
{
    private $age;
    private $name;
    private $sportsman;

    // Getters
    public function getName()
    {
        return $this->name;
    }

    public function getAge()
    {
        return $this->age;
    }

    // Issers
    public function isSportsman()
    {
        return $this->sportsman;
    }

    // Setters
    public function setName($name)
    {
        $this->name = $name;
    }

    public function setAge($age)
    {
        $this->age = $age;
    }

    public function setSportsman($sportsman)
    {
        $this->sportsman = $sportsman;
    }
}

Now, if you want to serialize this object into JSON, you only need to use the Serializer service created before:

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$person = new Acme\Person();
$person->setName('foo');
$person->setAge(99);
$person->setSportsman(false);

$jsonContent = $serializer->serialize($person, 'json');

// $jsonContent contains {"name":"foo","age":99,"sportsman":false}

echo $jsonContent; // or return it in a Response

The first parameter of the serialize() is the object to be serialized and the second is used to choose the proper encoder, in this case JsonEncoder.

Deserializing an Object

You'll now learn how to do the exact opposite. This time, the information of the Person class would be encoded in XML format:

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use Acme\Person;

$data = <<<EOF
<person>
    <name>foo</name>
    <age>99</age>
    <sportsman>false</sportsman>
</person>
EOF;

$person = $serializer->deserialize($data, Person::class, 'xml');

In this case, deserialize() needs three parameters:

  1. The information to be decoded
  2. The name of the class this information will be decoded to
  3. The encoder used to convert that information into an array

By default, additional attributes that are not mapped to the denormalized object will be ignored by the Serializer component. Set the allow_extra_attributes key of the deserialization context to false to let the serializer throw an exception when additional attributes are passed:

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$data = <<<EOF
<person>
    <name>foo</name>
    <age>99</age>
    <city>Paris</city>
</person>
EOF;

// this will throw a Symfony\Component\Serializer\Exception\ExtraAttributesException
// because "city" is not an attribute of the Person class
$person = $serializer->deserialize($data, 'Acme\Person', 'xml', array(
    'allow_extra_attributes' => false,
));

Deserializing in an Existing Object

The serializer can also be used to update an existing object:

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// ...
$person = new Person();
$person->setName('bar');
$person->setAge(99);
$person->setSportsman(true);

$data = <<<EOF
<person>
    <name>foo</name>
    <age>69</age>
</person>
EOF;

$serializer->deserialize($data, Person::class, 'xml', array('object_to_populate' => $person));
// $person = Acme\Person(name: 'foo', age: '69', sportsman: true)

This is a common need when working with an ORM.

Attributes Groups

Sometimes, you want to serialize different sets of attributes from your entities. Groups are a handy way to achieve this need.

Assume you have the following plain-old-PHP object:

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namespace Acme;

class MyObj
{
    public $foo;

    private $bar;

    public function getBar()
    {
        return $this->bar;
    }

    public function setBar($bar)
    {
        return $this->bar = $bar;
    }
}

The definition of serialization can be specified using annotations, XML or YAML. The ClassMetadataFactory that will be used by the normalizer must be aware of the format to use.

Initialize the ClassMetadataFactory like the following:

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use Symfony\Component\Serializer\Mapping\Factory\ClassMetadataFactory;
// For annotations
use Doctrine\Common\Annotations\AnnotationReader;
use Symfony\Component\Serializer\Mapping\Loader\AnnotationLoader;
// For XML
// use Symfony\Component\Serializer\Mapping\Loader\XmlFileLoader;
// For YAML
// use Symfony\Component\Serializer\Mapping\Loader\YamlFileLoader;

$classMetadataFactory = new ClassMetadataFactory(new AnnotationLoader(new AnnotationReader()));
// For XML
// $classMetadataFactory = new ClassMetadataFactory(new XmlFileLoader('/path/to/your/definition.xml'));
// For YAML
// $classMetadataFactory = new ClassMetadataFactory(new YamlFileLoader('/path/to/your/definition.yaml'));

Then, create your groups definition:

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    namespace Acme;
    
    use Symfony\Component\Serializer\Annotation\Groups;
    
    class MyObj
    {
        /**
         * @Groups({"group1", "group2"})
         */
        public $foo;
    
        /**
         * @Groups({"group3"})
         */
        public function getBar() // is* methods are also supported
        {
            return $this->bar;
        }
    
        // ...
    }
    
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    Acme\MyObj:
        attributes:
            foo:
                groups: ['group1', 'group2']
            bar:
                groups: ['group3']
    
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    <?xml version="1.0" ?>
    <serializer xmlns="http://symfony.com/schema/dic/serializer-mapping"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://symfony.com/schema/dic/serializer-mapping
            http://symfony.com/schema/dic/serializer-mapping/serializer-mapping-1.0.xsd"
    >
        <class name="Acme\MyObj">
            <attribute name="foo">
                <group>group1</group>
                <group>group2</group>
            </attribute>
    
            <attribute name="bar">
                <group>group3</group>
            </attribute>
        </class>
    </serializer>
    

You are now able to serialize only attributes in the groups you want:

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use Symfony\Component\Serializer\Serializer;
use Symfony\Component\Serializer\Normalizer\ObjectNormalizer;

$obj = new MyObj();
$obj->foo = 'foo';
$obj->setBar('bar');

$normalizer = new ObjectNormalizer($classMetadataFactory);
$serializer = new Serializer(array($normalizer));

$data = $serializer->normalize($obj, null, array('groups' => array('group1')));
// $data = array('foo' => 'foo');

$obj2 = $serializer->denormalize(
    array('foo' => 'foo', 'bar' => 'bar'),
    'MyObj',
    null,
    array('groups' => array('group1', 'group3'))
);
// $obj2 = MyObj(foo: 'foo', bar: 'bar')

Selecting Specific Attributes

It is also possible to serialize only a set of specific attributes:

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use Symfony\Component\Serializer\Serializer;
use Symfony\Component\Serializer\Normalizer\ObjectNormalizer;

class User
{
    public $familyName;
    public $givenName;
    public $company;
}

class Company
{
    public $name;
    public $address;
}

$company = new Company();
$company->name = 'Les-Tilleuls.coop';
$company->address = 'Lille, France';

$user = new User();
$user->familyName = 'Dunglas';
$user->givenName = 'Kévin';
$user->company = $company;

$serializer = new Serializer(array(new ObjectNormalizer()));

$data = $serializer->normalize($user, null, array('attributes' => array('familyName', 'company' => ['name'])));
// $data = array('familyName' => 'Dunglas', 'company' => array('name' => 'Les-Tilleuls.coop'));

Only attributes that are not ignored (see below) are available. If some serialization groups are set, only attributes allowed by those groups can be used.

As for groups, attributes can be selected during both the serialization and deserialization process.

Ignoring Attributes

Note

Using attribute groups instead of the setIgnoredAttributes() method is considered best practice.

As an option, there's a way to ignore attributes from the origin object. To remove those attributes use the setIgnoredAttributes() method on the normalizer definition:

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use Symfony\Component\Serializer\Serializer;
use Symfony\Component\Serializer\Encoder\JsonEncoder;
use Symfony\Component\Serializer\Normalizer\ObjectNormalizer;

$normalizer = new ObjectNormalizer();
$normalizer->setIgnoredAttributes(array('age'));
$encoder = new JsonEncoder();

$serializer = new Serializer(array($normalizer), array($encoder));
$serializer->serialize($person, 'json'); // Output: {"name":"foo","sportsman":false}

Converting Property Names when Serializing and Deserializing

Sometimes serialized attributes must be named differently than properties or getter/setter methods of PHP classes.

The Serializer Component provides a handy way to translate or map PHP field names to serialized names: The Name Converter System.

Given you have the following object:

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class Company
{
    public $name;
    public $address;
}

And in the serialized form, all attributes must be prefixed by org_ like the following:

{"org_name": "Acme Inc.", "org_address": "123 Main Street, Big City"}

A custom name converter can handle such cases:

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use Symfony\Component\Serializer\NameConverter\NameConverterInterface;

class OrgPrefixNameConverter implements NameConverterInterface
{
    public function normalize($propertyName)
    {
        return 'org_'.$propertyName;
    }

    public function denormalize($propertyName)
    {
        // remove org_ prefix
        return 'org_' === substr($propertyName, 0, 4) ? substr($propertyName, 4) : $propertyName;
    }
}

The custom normalizer can be used by passing it as second parameter of any class extending AbstractNormalizer, including GetSetMethodNormalizer and PropertyNormalizer:

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use Symfony\Component\Serializer\Encoder\JsonEncoder;
use Symfony\Component\Serializer\Normalizer\ObjectNormalizer;
use Symfony\Component\Serializer\Serializer;

$nameConverter = new OrgPrefixNameConverter();
$normalizer = new ObjectNormalizer(null, $nameConverter);

$serializer = new Serializer(array($normalizer), array(new JsonEncoder()));

$obj = new Company();
$obj->name = 'Acme Inc.';
$obj->address = '123 Main Street, Big City';

$json = $serializer->serialize($obj, 'json');
// {"org_name": "Acme Inc.", "org_address": "123 Main Street, Big City"}
$objCopy = $serializer->deserialize($json, Company::class, 'json');
// Same data as $obj

CamelCase to snake_case

In many formats, it's common to use underscores to separate words (also known as snake_case). However, PSR-1 specifies that the preferred style for PHP properties and methods is CamelCase.

Symfony provides a built-in name converter designed to transform between snake_case and CamelCased styles during serialization and deserialization processes:

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use Symfony\Component\Serializer\NameConverter\CamelCaseToSnakeCaseNameConverter;
use Symfony\Component\Serializer\Normalizer\ObjectNormalizer;

$normalizer = new ObjectNormalizer(null, new CamelCaseToSnakeCaseNameConverter());

class Person
{
    private $firstName;

    public function __construct($firstName)
    {
        $this->firstName = $firstName;
    }

    public function getFirstName()
    {
        return $this->firstName;
    }
}

$kevin = new Person('Kévin');
$normalizer->normalize($kevin);
// ['first_name' => 'Kévin'];

$anne = $normalizer->denormalize(array('first_name' => 'Anne'), 'Person');
// Person object with firstName: 'Anne'

Serializing Boolean Attributes

If you are using isser methods (methods prefixed by is, like Acme\Person::isSportsman()), the Serializer component will automatically detect and use it to serialize related attributes.

The ObjectNormalizer also takes care of methods starting with has, add and remove.

Using Callbacks to Serialize Properties with Object Instances

When serializing, you can set a callback to format a specific object property:

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use Acme\Person;
use Symfony\Component\Serializer\Encoder\JsonEncoder;
use Symfony\Component\Serializer\Normalizer\GetSetMethodNormalizer;
use Symfony\Component\Serializer\Serializer;

$encoder = new JsonEncoder();
$normalizer = new GetSetMethodNormalizer();

$callback = function ($dateTime) {
    return $dateTime instanceof \DateTime
        ? $dateTime->format(\DateTime::ISO8601)
        : '';
};

$normalizer->setCallbacks(array('createdAt' => $callback));

$serializer = new Serializer(array($normalizer), array($encoder));

$person = new Person();
$person->setName('cordoval');
$person->setAge(34);
$person->setCreatedAt(new \DateTime('now'));

$serializer->serialize($person, 'json');
// Output: {"name":"cordoval", "age": 34, "createdAt": "2014-03-22T09:43:12-0500"}

Normalizers

There are several types of normalizers available:

ObjectNormalizer

This normalizer leverages the PropertyAccess Component to read and write in the object. It means that it can access to properties directly and through getters, setters, hassers, adders and removers. It supports calling the constructor during the denormalization process.

Objects are normalized to a map of property names (method name stripped of the "get"/"set"/"has"/"remove" prefix and converted to lower case) to property values.

The ObjectNormalizer is the most powerful normalizer. It is configured by default when using the Symfony Standard Edition with the serializer enabled.

GetSetMethodNormalizer

This normalizer reads the content of the class by calling the "getters" (public methods starting with "get"). It will denormalize data by calling the constructor and the "setters" (public methods starting with "set").

Objects are normalized to a map of property names (method name stripped of the "get" prefix and converted to lower case) to property values.

PropertyNormalizer

This normalizer directly reads and writes public properties as well as private and protected properties. It supports calling the constructor during the denormalization process.

Objects are normalized to a map of property names to property values.

JsonSerializableNormalizer

This normalizer works with classes that implement JsonSerializable.

It will call the JsonSerializable::jsonSerialize() method and then further normalize the result. This means that nested JsonSerializable classes will also be normalized.

This normalizer is particularly helpful when you want to gradually migrate from an existing codebase using simple json_encode to the Symfony Serializer by allowing you to mix which normalizers are used for which classes.

Unlike with json_encode circular references can be handled.

DateTimeNormalizer
This normalizer converts DateTimeInterface objects (e.g. DateTime and DateTimeImmutable) into strings. By default it uses the RFC3339 format.
DataUriNormalizer
This normalizer converts SplFileInfo objects into a data URI string (data:...) such that files can be embedded into serialized data.
DateIntervalNormalizer

This normalizer converts DateInterval objects into strings. By default it uses the P%yY%mM%dDT%hH%iM%sS format.

New in version 3.4: The DateIntervalNormalizer normalizer was added in Symfony 3.4.

Encoders

The Serializer component supports many formats out of the box:

JsonEncoder
This class encodes and decodes data in JSON.
XmlEncoder
This class encodes and decodes data in XML.
YamlEncoder
This encoder encodes and decodes data in YAML. This encoder requires the Yaml Component.
CsvEncoder
This encoder encodes and decodes data in CSV.

All these encoders are enabled by default when using the Symfony Standard Edition with the serializer enabled.

Handling Circular References

Circular references are common when dealing with entity relations:

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class Organization
{
    private $name;
    private $members;

    public function setName($name)
    {
        $this->name = $name;
    }

    public function getName()
    {
        return $this->name;
    }

    public function setMembers(array $members)
    {
        $this->members = $members;
    }

    public function getMembers()
    {
        return $this->members;
    }
}

class Member
{
    private $name;
    private $organization;

    public function setName($name)
    {
        $this->name = $name;
    }

    public function getName()
    {
        return $this->name;
    }

    public function setOrganization(Organization $organization)
    {
        $this->organization = $organization;
    }

    public function getOrganization()
    {
        return $this->organization;
    }
}

To avoid infinite loops, GetSetMethodNormalizer throws a CircularReferenceException when such a case is encountered:

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$member = new Member();
$member->setName('Kévin');

$org = new Organization();
$org->setName('Les-Tilleuls.coop');
$org->setMembers(array($member));

$member->setOrganization($org);

echo $serializer->serialize($org, 'json'); // Throws a CircularReferenceException

The setCircularReferenceLimit() method of this normalizer sets the number of times it will serialize the same object before considering it a circular reference. Its default value is 1.

Instead of throwing an exception, circular references can also be handled by custom callables. This is especially useful when serializing entities having unique identifiers:

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$encoder = new JsonEncoder();
$normalizer = new ObjectNormalizer();

$normalizer->setCircularReferenceHandler(function ($object) {
    return $object->getName();
});

$serializer = new Serializer(array($normalizer), array($encoder));
var_dump($serializer->serialize($org, 'json'));
// {"name":"Les-Tilleuls.coop","members":[{"name":"K\u00e9vin", organization: "Les-Tilleuls.coop"}]}

Handling Serialization Depth

The Serializer component is able to detect and limit the serialization depth. It is especially useful when serializing large trees. Assume the following data structure:

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namespace Acme;

class MyObj
{
    public $foo;

    /**
     * @var self
     */
    public $child;
}

$level1 = new MyObj();
$level1->foo = 'level1';

$level2 = new MyObj();
$level2->foo = 'level2';
$level1->child = $level2;

$level3 = new MyObj();
$level3->foo = 'level3';
$level2->child = $level3;

The serializer can be configured to set a maximum depth for a given property. Here, we set it to 2 for the $child property:

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    use Symfony\Component\Serializer\Annotation\MaxDepth;
    
    namespace Acme;
    
    class MyObj
    {
        /**
         * @MaxDepth(2)
         */
        public $foo;
    
        // ...
    }
    
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    Acme\MyObj:
        attributes:
            foo:
                max_depth: 2
    
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    <?xml version="1.0" ?>
    <serializer xmlns="http://symfony.com/schema/dic/serializer-mapping"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://symfony.com/schema/dic/serializer-mapping
            http://symfony.com/schema/dic/serializer-mapping/serializer-mapping-1.0.xsd"
    >
        <class name="Acme\MyObj">
            <attribute name="foo">
                <max-depth>2</max-depth>
            </attribute>
    </serializer>
    

The metadata loader corresponding to the chosen format must be configured in order to use this feature. It is done automatically when using the Symfony Standard Edition. When using the standalone component, refer to the groups documentation to learn how to do that.

The check is only done if the enable_max_depth key of the serializer context is set to true. In the following example, the third level is not serialized because it is deeper than the configured maximum depth of 2:

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$result = $serializer->normalize($level1, null, array('enable_max_depth' => true));
/*
$result = array(
    'foo' => 'level1',
    'child' => array(
            'foo' => 'level2',
            'child' => array(
                    'child' => null,
                ),
        ),
);
*/

Handling Arrays

The Serializer component is capable of handling arrays of objects as well. Serializing arrays works just like serializing a single object:

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use Acme\Person;

$person1 = new Person();
$person1->setName('foo');
$person1->setAge(99);
$person1->setSportsman(false);

$person2 = new Person();
$person2->setName('bar');
$person2->setAge(33);
$person2->setSportsman(true);

$persons = array($person1, $person2);
$data = $serializer->serialize($persons, 'json');

// $data contains [{"name":"foo","age":99,"sportsman":false},{"name":"bar","age":33,"sportsman":true}]

If you want to deserialize such a structure, you need to add the ArrayDenormalizer to the set of normalizers. By appending [] to the type parameter of the deserialize() method, you indicate that you're expecting an array instead of a single object.

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use Symfony\Component\Serializer\Encoder\JsonEncoder;
use Symfony\Component\Serializer\Normalizer\ArrayDenormalizer;
use Symfony\Component\Serializer\Normalizer\GetSetMethodNormalizer;
use Symfony\Component\Serializer\Serializer;

$serializer = new Serializer(
    array(new GetSetMethodNormalizer(), new ArrayDenormalizer()),
    array(new JsonEncoder())
);

$data = ...; // The serialized data from the previous example
$persons = $serializer->deserialize($data, 'Acme\Person[]', 'json');

Recursive Denormalization and Type Safety

The Serializer Component can use the PropertyInfo Component to denormalize complex types (objects). The type of the class' property will be guessed using the provided extractor and used to recursively denormalize the inner data.

When using the Symfony Standard Edition, all normalizers are automatically configured to use the registered extractors. When using the component standalone, an implementation of PropertyTypeExtractorInterface, (usually an instance of PropertyInfoExtractor) must be passed as the 4th parameter of the ObjectNormalizer:

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use Symfony\Component\PropertyInfo\Extractor\ReflectionExtractor;
use Symfony\Component\Serializer\Serializer;
use Symfony\Component\Serializer\Normalizer\DateTimeNormalizer;
use Symfony\Component\Serializer\Normalizer\ObjectNormalizer;

namespace Acme;

class ObjectOuter
{
    private $inner;
    private $date;

    public function getInner()
    {
        return $this->inner;
    }

    public function setInner(ObjectInner $inner)
    {
        $this->inner = $inner;
    }

    public function setDate(\DateTimeInterface $date)
    {
        $this->date = $date;
    }

    public function getDate()
    {
        return $this->date;
    }
}

class ObjectInner
{
    public $foo;
    public $bar;
}

$normalizer = new ObjectNormalizer(null, null, null, new ReflectionExtractor());
$serializer = new Serializer(array(new DateTimeNormalizer(), $normalizer));

$obj = $serializer->denormalize(
    array('inner' => array('foo' => 'foo', 'bar' => 'bar'), 'date' => '1988/01/21'),
     'Acme\ObjectOuter'
);

dump($obj->getInner()->foo); // 'foo'
dump($obj->getInner()->bar); // 'bar'
dump($obj->getDate()->format('Y-m-d')); // '1988-01-21'

When a PropertyTypeExtractor is available, the normalizer will also check that the data to denormalize matches the type of the property (even for primitive types). For instance, if a string is provided, but the type of the property is int, an UnexpectedValueException will be thrown. The type enforcement of the properties can be disabled by setting the serializer context option ObjectNormalizer::DISABLE_TYPE_ENFORCEMENT to true.

Learn more

A popular alternative to the Symfony Serializer Component is the third-party library, JMS serializer (released under the Apache license, so incompatible with GPLv2 projects).

This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.